1. Field of the Invention
The present invention relates to an electroluminescence element (hereinafter referred to as EL element) used for a planar light emitter, a display device, and the like, and more particularly, to an EL element in which a light scattering film is provided near a light emitting layer to improve light emission efficiency.
2. Description of the Related Art
Up to now, an inorganic EL element including a light emitting layer made of an inorganic material has been known. In recent years, an organic EL element using a low-molecular organic film or a polymer organic film as the light emitting layer has been under development. The EL elements have a feature that a device is thin and light is emitted at low power consumption. Therefore, the EL elements have been put into practical use for a planar light emitter for illuminating a liquid crystal display. In addition, the EL elements have been under research and development for a full color image display device in which pixels are arranged in dot matrix.
FIG. 13 shows across sectional structure of a full color display type EL element which is known up to now. As shown in FIG. 13, rear electrodes 52 are formed on a glass substrate 51. An insulating layer 53, three light emitting layers 54, 55, and 56, a display-side insulating layer 57, and transparent electrodes 58 are stacked on the rear electrodes 52 in this order. Each of the rear electrodes 52 is a stripe electrode extending in a direction parallel to the paper surface of FIG. 13. Each of the transparent electrodes 58 is a stripe electrode extending in a direction perpendicular to the paper surface of FIG. 13. A color filter layer 59 is formed to cover the transparent electrodes 58. The three light emitting layers 54, 55, and 56 are a green light emitting layer of ZnS:Tb, F, a blue-green light emitting layer of SrS:Ce, and a yellow-orange light emitting layer of ZnS:Mn, respectively, which are stacked in this order. The color filter layer 59 includes a red light transmission filter 59r, a green light transmission filter 59g, and a blue light transmission filter 59b, which are separated from one another and formed to cover the transparent electrodes 58. Such the structure is described in, for example, JP 01-315988 A.
In the EL element, a driving voltage is applied between each of the rear electrodes 52 and each of the transparent electrodes 58 to emit light from each of the light emitting layers 54, 55, and 56 located at intersections therebetween. Green (hereinafter referred to as G) light, red-orange (hereinafter referred to as R) light, and blue (hereinafter referred to as B) light are simultaneously emitted from the light emitting layers, so white light is obtained. When the white light reaches the color filter layer 59, light of a color other than a corresponding color of the color filter layer 59 is absorbed for each of pixels located at the intersections and only light of the corresponding color of the color filter layer 59 passes therethrough. Therefore, much of emitted light is absorbed by the color filter layer 59.
Actual scan-driving is performed by line-sequentially selecting a large number of rear electrodes 52 or a large number of transparent electrodes 58 which are provided in a stripe shape and separated from one another. That is, assume that the transparent electrodes 58 are used as scanning electrodes and the rear electrodes 52 are used as signal electrodes. In this case, while one of the transparent electrodes 58 is selected, signal voltages are supplied to the rear electrodes 52 to emit light from a light emitting layer located at each intersection portion between the selected transparent electrode and each of the rear electrodes 52. Such an operation is sequentially repeated for scanning, thereby realizing full color display.
Light generated in a light emitting layer includes not only light emitted from the surface of the light emitting layer in a perpendicular direction but also light confined in the light emitting layer. For example, when the light emitting layer 54 (or light emitting layer 56) has a larger refractive index to visible light than the insulating layer 53 (or display-side insulating layer 57) in the above-mentioned conventional structure, light in a lateral direction is totally reflected at an interface between the insulating layer and the light emitting layer. Therefore, the light cannot be taken out from the light emitting layer, so the light is confined in the light emitting layer. Thus, there is a problem in that the light emission efficiency of the EL element used for the planar light emitter reduces. Even when the refractive indexes of the light emitting layers 54, 55, and 56 and the insulating layer 53 (or display-side insulating layer 57) become lower toward the outside, the generated light is likely to be confined in each of the light emitting layers. This causes a reduction in light emission efficiency of the EL element used for the planar light emitter.
In the above-mentioned known example used for the full color display device, much of light emitted from the light emitting layers 54, 55, and 56 is absorbed by the color filter layer 59. For example, when the color filter layer 59 is caused to transmit a blue light beam, a green light beam and a red-orange light beam are absorbed thereby, and when the color filter layer 59 is caused to transmit the red-orange light beam, the blue light beam and the green light beam are absorbed thereby. Therefore, there is a problem in that the amount of light taken out for display necessarily reduces.